The present invention relates to an ink composition with which the indoor storage stability (gas resistance) of an image produced from the composition can be increased, and more particularly relates to a phthalocyanine-based cyan dye-containing ink composition with which color fading of an image can be effectively prevented.
Ink compositions featuring a phthalocyanine-based cyan dye have been widely used in the past as ink compositions for producing images with excellent cyan color and so forth. Also, it has now become very popular to form images by using an ink composition that produces such a colored image along with one or more ink compositions that produce other colors, and to display the resulting images composed of a plurality of colors (color images) indoors and elsewhere.
However, with an ink composition made with a phthalocyanine-based cyan dye, because phthalocyanine-based cyan dyes have poor resistance to oxidative gases such as ozone, sulfur oxides, and nitrogen oxides that are present in the air, and particularly poor ozone resistance, when a multicolor image is formed using this cyan dye along with ink compositions of other colors, and this image is displayed indoors, the image formed from the cyan dye fades faster than the images of other colors.
Various ink compositions have been developed in an effort to prevent this color facing of phthalocyanine. For instance, WO99/50363 discloses as a means for increasing light fastness a cyan ink composition containing a copper phthalocyanine-based cyan dye, an imidazole derivative, and either an antioxidant, a saccharide, or naphthalene-1-sulfonic acid having an amino group or a hydroxyl group in the 4-position. With this ink composition, however, its resistance to ozone gas has not been evaluated, and the molecules of the components used in the composition include hydroxyl groups and amino groups, which cause the image to turn green or black as a result of nitrogen oxides (NOx gas).
The inventors conducted diligent research into these problems, and as a result found that the ozone resistance of a recorded image can be effectively increased, without the image changing color or fading as a result of nitrogen oxides, by adding a specific aromatic sulfonic acid and/or a salt thereof to an ink composition in which a phthalocyanine-based cyan dye is used (Japanese Patent Applications 2003-341633 and 2004-31869). However, further research has revealed that the additives in an ink composition to which these additives have been added sometimes precipitate on the surface of the image when the recorded image is stored at room temperature for an extended period. This was seen, for example, when a sodium salt or potassium salt of this aromatic compound was used, such as a disodium salt of naphthalene-1,5-disulfonic acid. This precipitation of additives was not seen when the aromatic sulfonate was contained in a small amount in the ink, but the problem was that the ozone resistance decreased as the additive amount was reduced.
Furthermore, bronzing was sometimes noted when an image was printed in an environment of high temperature and humidity. Here again, no bronzing was seen when the aromatic sulfonate was contained in a small amount in the ink, but the problem was that the ozone resistance decreased as the additive amount was reduced.
Therefore, it is an object of the present invention to provide an ink composition with which the indoor storage stability (gas resistance) of an image produced with an ink composition featuring a phthalocyanine-based cyan dye can be increased without causing any additive precipitation or bronzing.
As a result of diligent research, the inventors discovered that the stated object could be achieved by an ink composition containing a specific phthalocyanine-based cyan dye along with other specific components.